This review addresses the development trends of dielectric ceramics, the key material for Multilayer Ceramic Capacitors (MLCCs), which are essential components in high-performance electronic devices. Traditional MLCCs have employed BaTiO₃ (BT)-based dielectrics to achieve high dielectric constant and low resistance. By minimizing oxygen vacancies and suppressing grain growth in BT materials, the temperature and voltage stability of MLCCs have been improved, leading to the development of MLCCs with diverse properties. However, the maximum dielectric constant of approximately 3000 in BT materials poses a limitation in overcoming the trade-off between rated voltage and capacitance density. Therefore, ultra-high permittivity dielectric materials have gained attention to meet the requirements of ultra-high-performance MLCCs, and ongoing research focuses on enhancing the temperature and frequency stability of these materials. This review analyzes the characteristics and limitations of conventional BT materials and explores recent research trends and future potential in developing new MLCCs based on ultra-high dielectric constant materials.

Phase evolution, sintering behavior, microstructure, and microwave dielectric properties of (1-x) mol Ba₃V₄O₁₃ - (x) mol BaV₂O₆ system were investigated. The sintered specimens of all compositions consisted of Ba₃V₄O₁₃ and BaV₂O₆, and no secondary phase was observed. As x increased, the linear shrinkage decreased to the composition of x=0.5, and then increased again, implying that Ba₃V₄O₁₃ and BaV₂O₆ phases interfered mutually with each other during sintering. All compositions showed a dense microstructure with a large grain growth. Cracks were observed in some compositions because of the relatively high sintering temperature of 620~640℃. As x increased, the dielectric constant increased, while the quality factor was maintained from about 50,000 GHz to about 70,000 GHz up to the composition of x=0.9, and then decreased to 20,987~27,180 GHz at the composition of x=1.0. As x increased, the temperature coefficient of the resonance frequency showed a (+) value from a (-) value. The dielectric constant, the quality factor, and the temperature coefficient of resonant frequency of x=0.7 composition sintered at 640℃ for 4 hours were 10.61, 71,126 GHz, and -4.9 ppm/℃, respectively. This composition showed a good chemical compatibility with Al powder, indicating that the Ba₃V₄O₁₃-BaV₂O₆ ceramics are a candidate material for ULTCC (Ultra-Low Temperature Co-fired Ceramics) applications.

BaTiO₃ powder was synthesized by a solid-state reaction using BaCO₃ and TiO₂. Different calcination temperatures (800℃, 850℃, 900℃, and 950℃) were set to investigate their effects on the properties of BaTiO₃ powder. The synthesized BaTiO₃ phase was confirmed to be a single phase by XRD, and the tetragonality (c/a) and crystallite size were calculated. Thereafter, each calcinated BaTiO₃ was sintered at five different sintering temperatures (1,100℃, 1,150℃, 1,200℃, 1,250℃, and 1,300℃), and the tetragonality, density, porosity, dielectric constant, and grain size were measured. As the calcination temperature increased, the tetragonality and crystallite size also increased, to 1.008 and 66 nm, respectively, at 950℃. Moreover, most pellets showed increased density, dielectric constant, and tetragonality as the sintering temperature increased up to 1,250℃; the same parameters slightly decreased at 1,300℃. It is noteworthy that the tetragonality of BaTiO₃ at 1,250℃ exhibits a very high c/a value of 1.0084. In addition, the grain size and dielectric constant measured near the Curie temperature increased as the sintering temperature increased.

The phase evolution, microstructure, and microwave dielectric properties of Ba(Mg_0.5-2xY_2xW_0.5-xTi_x)O₃ (x＝ 0.005~0.05) ceramics sintered at 1,700℃ for 1h were investigated. All compositions exhibited a 1:1 ordered cubic perovskite structure. The field emission scanning electron microscopy image revealed a dense microstructure in all the compositions. As the value of x increased, the lattice parameter, dielectric constant, and quality factor increased. The temperature coefficient of resonant frequency changed from -19.6 ppm/℃ to -5.9 ppm/℃ with increasing x value. The dielectric constant, quality factor, and temperature coefficient of resonant frequency of Ba(Mg_0.40Y_0.10W_0.45Ti_0.05)O₃ were 21.7, 132,685 GHz, and -5.9 ppm/℃, respectively.

Sintering, microstructure, thermal conductivity and microwave dielectric properties of xLiF-(1-x)MgO ceramics (x=0.03-0.10 mol) were investigated. The high density was obtained in the specimens of x≥0.06, i.e., 0.04 LiF-0.96 MgO in mol, whereas the amount of 0.03 mol LiF was insufficient to densify. From the result that the contact flattening in the sintered specimen was observed, the densification occurred through the liquid-phase sintering. The specimen of x=0.06 showed the highest room-temperature thermal conductivity. Relative density, thermal conductivity, dielectric constant, and quality factor (Q x f) of the specimen for x=0.06 sintered at 900°C for 4 h were 97.8%, 39.2 Wm-1K-1, 9.45, and 14,671 GHz, respectively.

Sintering and microwave dielectric properties of Zn2-2xSil+xO4 (x=O-0.10) ceramics were investigated. The secondary phase of ZnO was observed in the specimen for x=O whereas SiO2 was detected in that for x=0.05. The composition of Zn2SiO4 might be close to x=0.02, i.e., Zn1.96Si1.02O4; the ratio of Zn/Si is 1.922. The insufficient grain growth was observed in the specimen of x=O. For the specimens of x≥0.05 , the grain growth sufficiently occurred through the liquid phase sintering. The value of quality factor of all specimens was dependent on the x value, i.e., the ratio of Zn/Si, whereas that of dielectric constant was independent. Relative density, dielectric constant, and quality factor (Q×f) of the specimen for x=0.05, i.e., Znl.9Si1.05O4, sintered at 1,400℃ were 96.5%, 6.43, and 115,166 GHz, respectively.

In this study, lead-free Piezoelectric (Na0.47K0.47Sr0.03Ca0.03)(Nb0.94Ti0.06)O3-0.1 MnO2 ceramics werefabricated using mixed oxide method and the effects of various sintering temperature on the structural andelectrical properties were investigated. For the (Na0.47K0.47Sr0.03Ca0.03)(Nb0.94Ti0.06)O3-0.1 MnO2 (NKN-SCT-MnO2)ceramics sintered at temperatures of 1,025∼1,100℃. The results indicated that all specimens were perovskitesingle phase formation without any second phase. It has been shown that relative density is increased toincreasing sintering temperature. When the sintered temperature at 1,075℃, highest sintered density andmaximum value of 4.45 g/cm3. Average grain size is increased to increasing sintering temperature. Theelectromechanical coupling factor, dielectric constant, dielectric loss, d33 and curie temperature at the sinteringtemperature 1,075℃ of NKN-SCT-MnO2 specimens were 0.22, 511, 0.033, 103 and 380℃, respectively.

PTFE composites for use of microwave substrate were fabricated by impregnation and heat treatment fabrication with glass fabric. This study shows dielectric properties such as dielectric constant and loss can be controlled by thickness of PTFE composite with of pressure condition in heating press process. The dielectric constant of the PTFE composites has decreasing tendency as given higher pressure condition. The dielectric loss has similar result too. Especially, the case of the dielectric loss was affected by the condition of pressure at heating press and had the best performance under 3 MPa. In order to see the reason why thickness conditions make different, their microstructures were also observed.

The Sr0.7Bi2.3Nb2O9(SBNO) thin films were deposited on Si substrate by RF magnetron sputtering method at 300℃ of substrate temperature. And the SBNO thin films were annealed at 650～800℃ using RTA (rapid thermal annealing). The grain of SBNO thin films were increased with the increase of annealing temperature. The dielectric constant (100) of SBNO thin film was obtained by RTA above 750℃. The voltage dependence of dielectric loss showed a value within 0.03 in voltage ranges of -5～＋5 V. Also, the dielectric constant characteristics showed a stable value with the increase of frequency.

In this study, lead-free (Na0.465K0.465Bi0.07)(Nb0.93Ti0.07)O3-0.08MnO2 ceramics were fabricated by conventional mixed oxide method. Structural and electrical properties of lead-free (Na0.465K0.465Bi0.07)(Nb0.93Ti0.07)O3-0.08MnO2 ceramics with the variation of sintering temperature were investigated. As results of x-ray diffraction analysis, all specimens showed a typical polycrystalline perovskite structure without presence of the second phase. Sintered density increased with an increases of sintering temperature and the specimen sintered at 1,020℃ showed the maximum value of 4.5 g/cm3. The average grain size of the (Na0.465K0.465Bi0.07)(Nb0.93Ti0.07)O3-0.08MnO2 specimen sintered at 1,020℃ is about 0.83 μm. Electromechanical coupling factor, relative dielectric constant and dielectric loss of (Na0.465K0.465Bi0.07)(Nb0.93Ti0.07)O3-0.08MnO2 specimens sintered at 1,020℃ were 0.252, 741 and 0.043% respectively.

We investigated the dielectric and mechanical properties of ceramic polymer composite xBNT - (1-x)LCP (x= 0, 10, 20, 30, 40 vol.%). The disk shaped BNT (BaNd2Ti4O12) - LCP (liquid crystal polymer) composite samples were prepared by compression molding method. With increasing the BNT content in composites from 10 to 40 vol.%, the dielectric constant increased but the dielectric loss as well as bending strength of composites reduced. These composites were well described with modified Lichtenecker`s model having k = 0.392 and 0.303 for the first and second ball milled BNT filled composites, which means that the BNT filler in composites are well dispersed. The dielectric constant of the composite comprised of the second milled BNT (D50 = 1.39 um) was higher that of the composite of the first milled BNT (D50= 2.45 um), which seems to be related with the different particle size and dispersion of BNT fillers in LCP matrix. The bending strength of the composite containing the second milled BNT was superior to that of the composite of the first milled BNT.